Secondary air control arrangement for fuel oil burner

ABSTRACT

A cylindrical blast tube which defines an axial combustion air flow path of a modulating fuel burner is provided with a cylinder of smaller diameter, concentrically positioned within the tube to divide the air flow into an inner primary air flow path surrounding the fuel nozzle at the downstream end of the blast tube, and a secondary air path between cylinders. Air flow control means within the secondary air flow path modulates the flow of secondary air in response to burner demand during burner operation. The secondary flow modulating means may take the form of a series of circumferentially adjacent hinged vanes movable to flow blocking position; relatively rotatable matched perforated annular plates or cylinders relatively slidable with matched radial extensions.

Elites Runninger States Patent [191 [451 Nov. 11, 1975 [75] Inventor: Frederick L. Runninger,

Harrisonburg, Va.

[73] Assignee: Dunham-Bush, lnc.. Harrisonburg,

[22] Filed: Oct. 31, 1974 [21] Appl. No.: 519.708

Related US. Application Data [631 Continuation-impart of Ser. No. 395.882. Sept. 10.

1973. abandoned.

[52] US. Cl. 431/184 [51] Int. Cl. F23M 9/00 [58] Field of Search 431/182. 183. 184. 265. 431/12: 239/4164 [56] References Cited UNITED STATES PATENTS 3.049.173 8/1962 Costello et al 431/265 X 3.284.008 11/1966 Miller 431/12 X 3.486.834 12/1969 Frey et a1 431/12 Primary E.\'aminerEdward G. Favors Attorney, Agent, or Firm-Sughrue, Rothwell. Mion, Zinn & Macpeak [57 ABSTRACT A cylindrical blast tube which defines an axial combustion air flow path of a modulating fuel burner is provided with a cylinder of smaller diameter, concentrically positioned within the tube to divide the air flow into an inner primary air flow path surrounding the fuel nozzle at the downstream end of the blast tube. and a secondary air path between cylinders. Air flow control means within the secondary air flow path modulates the flow of secondary air in response to burner demand during burner operation. The secondary flow modulating means may take the form of a series of circumferentially adjacent hinged vanes movable to flow blocking position; relatively rotatable matched perforated annular plates or cylinders relatively slidable with matched radial extensions.

4 Claims, 9 Drawing Figures US. Patent Nov. 11,1975 Sheet 1 of3 3,918,886

US. Patent Nov. 11, 1975 Sheet 2 of3 3,918,886

US. Patent Nov. 11, 1975 Sheet 3 of 3 SECONDARY AIR CONTROL ARRANGEMENT FOR FUEL OIL BURNER This is a continuation in part application of US. application Ser. No. 395,882 filed Sept. 10, 1973, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to fuel oil burners for furnaces and the like, and more particularly to an arrangement for modulating the flow of combustion air through the oil burner blast tube.

2. Description of the Prior Art Furnaces which employ liquid fuel oil burners normally incorporate means for delivering combustion air to the vicinity of the fuel nozzle under pressure to promote efficient combustion of the resulting fuel air mixture. Combustion air for modulating such oil burners is normally controlled either on the inlet to the fan or the outlet from the fan remote from the area of combustion.

Where means are provided for the modulation of the combustion air around the nozzle at the cone or the, fire ring, these means are relatively inaccessible and once set remain fixed while the burner is on high or low fire. As a result, the velocity of the air leaving the cone or fire ring of the burner varies considerably from high fire to low fire. At low fire, frequently the velocity of the combustion air is not sufficient to provide good mixing of the fuel and the air, resulting in a smokey fire even though there is air in excess of that required for complete combustion. Under these conditions, carbon can deposit and pulsation is frequently a problem.

SUMMARY OF THE INVENTION The present invention comprises a secondary combustion air control system for improving a fluid fuel burner which insures the supply of requisite air for complete combustion when fired into a chamber against either positive or negative pressure. The secondary air modulating arrangement is incorporated within the burner structure and acts to vary the free area for passage of the secondary combustion air at the cone or fire ring such that the velocity of the combustion air remains approximately the same between high fire and low fire, even though the volume of air varies.

In particular, the present invention is directed to a modulating fuel burner for a furnace or the like having a first cylinder or blast tube forming an axial combustion air flow path with a second cylinder of smaller diameter and of less length concentrically positioned within the first tube at its upstream end and defining internally an inner, primary air flow path and an outer, secondary air flow path between the cylinders. The fuel nozzle and the electrodes for firing the fuel air mixture are carried axially within the second cylinder. The invention resides in means carried by the second cylinder for modulating secondary air flow between cylinders in response to burner demand. Such means may take the form of a series of hinged vanes in terms of a circumferential array which spans the secondary air flow path between cylinders, when pivoted radially to full flow blocking position, but which may be pivoted to a position generally in line with the secondary air flow so as to vary the cross sectional area of the secondary air flow path. Alternatively, the secondary air flow path may be variably changed by means of angularly shiftmeans operatively connect the modulating means for modulating the secondary air flow to the modulating motor which also controls fuel input through a modulating valve, with the modulating motor being responsive to burner demand changes during burner operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a fuel burner incorporating one form of the secondary air modulating arrangement of the present invention.

FIG. 2 is a sectional view of a portion of the downstream end of the blast tube of the fuel burner of FIG. 1 showing a modified form of fire ring.

FIG. 3 is a similar sectional view of a portion of the blast tube of FIG. 1, similar to that of FIG. 2 and showing yet another modification to the fire ring.

FIG. 4 is an end elevational view of the fuel burner of FIG. 1.

FIG. 5 is a cross sectional view of a fuel burner incorporating an alternate form of the secondary air modulating arrangement of the present invention.

FIG. 6 is a front elevational view of a fuel burner incorporating yet another form of the secondary air modulating arrangement of the present invention.

FIG. 7 is a sectional view of a portion of the fuel burner shown in FIG. 6.

FIG. 8 is a front end view of a portion of the fuel burner of FIG. 1 showing the fuel and secondary air modulating motor and the connection to the actuator shaft and the fuel return valve.

FIG. 9 is a plan view of the portion of the fuel burner illustrated in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown in section a liquid fuel burner indicated generally at 10 which may be used in conjunction with a furnace or the like and which incorporates the secondary air modulating arrangement of the present invention in one form, purposely only those elements of the fuel burner necessary to a full description and understanding of the present invention are shown in the drawings. In that respect, a wall 12 defines to the left, FIG. 1, a combustion chamber 14 with the blast tube 16 in the form of a first cylinder extending through an opening 18 within that wall and mounted thereto by means of a mounting ring 20. The blast tube or cylinder 16 defines a combustion air flow path within the same, the air flow entering the upstream end of the tube and passing from right to left, exiting into the combustion chamber 14 at the downstream end from fire ring 22 which concentrically surrounds the cylinder 16 at the left end of the fuel burner, FIG. 1. A further tubular casing 24 is fixed at its flanged end 26 to cylinder flange 28 at the upstream end of the tube 16, the two flanges 26 and 28 being coupled together by bolts and nuts or the like (not shown). With the exception of particular controls pertinent to the present invention, the other structural components of the fuel burner are not illustrated, nor discussed. in conventional fashion, the fuel burner of the present invention incorporates a second cylinder 30 of smaller diameter than cylinder 16 which is concentrically positioned therein and which separates the combustion air flow path through the first cylinder 16 into a primary air flow path through the interior of cylinder 30 and a secondary air flow path 34 between cylinders 16 and 30. Mounted within the inner cylinder 30, is a fuel nozzle 36 coupled to a fuel supply line or conduit 38', the nozzle being supported within the cylinder 30 by means of one or more cross pieces 40 and being fixed axially with respect to the burner. A pair of electrodes 42 are disposed on respective sides of the fuel nozzle 36 and fixedly maintained in position by means of insulators 44 which pass through and are affixed to the cross piece 40. The electrodes 42 are energized for spark discharge thereacross by means of an ignition coil or the like indi cated at 45 with the electrical leads 46 extending from the coil 45 to respective electrodes. Upstream of the cylinder 30, there is positioned a primary air flow control disc assembly 48 which includes means (not shown) such as variably sized openings therein for varying the flow of primary air through cylinder 30. The primary air control assembly 48 acts in conjunction with the radial vane diffuser 50 at the downstream end of cylinder 30 to control and assist in mixing the primary air stream with the fuel as the fuel is discharged from the downstream end of nozzle 36. In this respect, reference to FIG. 4 illustrates the diffuser 50 as comprising a series of radial vanes 52 whose edges overlap, with the vanes being twisted about the radial axis so as to form primary air flow passages between the vanes, the vanes 52 also preferably being curved about the radial axis to effect a swirling motion to the primary air as it passes through the diffuser 50. An irregular opening 54 is defined centrally of the diffuser by the radial inner ends of vanes 52, permitting primary air to exit from the cylinder 30 through the vanes and also centrally through the opening 54. The radial vanes of the diffuser remain set after manufacture and the flow of primary air is not varied by this means except as it may be modulated in terms of volume by varying blower speed, etc., although adjustment of the primary air control disc assembly 48 results in more or less primary air flowing through cylinder 30 for a given fan or blower output. With respect to the fan or blower (not shown) associated with the housing of fuel burner 10, it is assumed that under certain conditions the fan or blower velocity remains constant directing combustion air at a given flow rate to casing 24 which leads directly into the blast tube or cylinder 16.

The present invention is directd to the flow restriction means for modulating the secondary air flow for the fuel burner 10. With respect to the embodiment of FlGS. l and 4, the invention consists primarily in providing a circumferential array of pivotable vanes which are disposed within the annular secondary air flow path 34 between the blast tube or cylinder 16 and cylinder 30. As seen best in FIG. 4. a circular array of sheet metal hinges 56 are provided at spaced circumferential positions about the periphery of cylinder 30, the hinges 56 carrying alternately. driving or actuator vanes 58 and driven or follower vanes 60. The actuator vanes 58 are of irregular rectangular form in plan view. their radially outermost edges 62 being curved to conform to the periphery of cylinder 16 and being positively pivoted into vertical or radially upright position such that the outer edgds 62 tend to contact a fixed angular ring 64 which acts as a fixed stop. In contrast, the follower vanes 60 are somewhat fan shaped such that their side edges 66 are inclined outwardly away from their inner edge 68. Again, the radially outermost edge 70 is curved and conforms generally to the inner periphery of cylinder 16, and engages ring 64 when moved into radially upright position. The inner edges of both vanes are fixed to the hinges 56 such that they rotate about axes at right angles to radii bisecting the vanes. Further, the follower vanes 60 are of such a size that their side edges 66 overlap side edges 72 of the driving or actuator vanes 58. Since the combustion air flows under pressure through the blast tube 16, the resulting air pressure is sufficient to cause the follower vanes 60 to move from their radial position to their position generally in line with the axis of the tube and follow pivoting of the driving vanes from their full radial position. Regardless of the angular position of the driving vanes 58, the follower vanes 60 will always be in edge contact with the driving vanes due to the edge overlap. This reduces the number of actuators necessary to effect control of the secondary air flow in passage 34.

Preferably, the burner 10 is provided with a positive drive system which in the illustrated form, FIG. 1, comprises a limited rotation electric motor 71 mounted to a fuel burner component 76 which lies outside of tube 24. The electric motor is supplied with an electrical control signal indicative of burner demand to rotate shaft 73 through a limited angle, on the order of 90, which rotates the enlarged diameter portion 75 to the same extent about the axis of shaft 73. An L-shaped arm 77 is welded to or otherwise affixed to the enlarged diameter portion 75 of shaft 73 and thus it rotates in the same limited arc. Mounted to arm 77 is a universal joint element 79 to which one end of a rod or link 81 is coupled, the opposite end of the rod 81 being coupled to a similar universal joint element 83 which in turn is mounted to arm 85 supported by shaft 86. Shaft 86 is mounted to cylinder 24 by bushings 88 and projects therethrough for rotation about the shaft axis to the degree determined by rotation of motor shaft 73. In addition to arm 85 being fixedly mounted to shaft 86, the shaft has further affixed thereto a second arm 87 which rotates therewith, the arm 87 supporting in turn a universal joint element 89 to which is fixed one end of rod 91. The opposite end of the rod 91 is further fixed by way of a universal joint connection or element 93 to arm 82 such that when the arm 87 rotates about the axis of shaft 86, arm 82 rotates correspondingly about a pivot axis defined by pin 105 which forms a pin connection between arm 82 and a tubular support member 80 which protrudes from one side of a control valve 72. A cam surface 107 on arm 82 impinges against a cam follower 109, FIG. 1, which mechanically modulates the flow of fuel which enters valve 72 through fuel line 101 and leaves through fuel line 78. ln this respect, valve 72 constitutes a fuel oil return control valve. Fuel oil is delivered to the fuel nozzle 36 by way of a supply line 93 which connects to a fuel onoff control valve 97 with fuel line 95 leading from the on-off valve 97 to fuel burner nozzle inlet line 38. In the illustrated embodiment of the invention. not all of the fuel is necessarily ejected from burner 36 but some of it is returned by way of control valve 72, thereby modulating the supply of fuel to the burner nozzle 36 in response to burner demand during burner operation. The fuel which is not burned, that is, not ejected from nozzle 36 returns by way of return tube 39 and fuel return line 101 to the inlet side of control valve 72 with the flow rate returning to the supply line 93 determined by the angular position of shaft 86 which controls the position of the cam surface 107 relative to cam follower 109. This type of valve is conventional, and by energization of the limited rotation elecric motor 71, less fuel is returned by return lines 78 and 99 to the fuel supply side of the burner nozzle 36 as burner demand increases during burner operation. Likewise, rotation of shaft 86 effects an increase in flow of secondary air as burner demand increases during burner operation. In this respect, rotation of actuator shaft 86 causes a series of vane control rods 90 to shift axially simultaneously, the ends of the control rods remote from the actuator shaft 86 being connected via ball and socket couplings 92 to the hinges 56 and the other ends of rods 90 being coupled to shaft 86 by similar couplings 94. Movement of the rods 90 to the left in FIG. 1 causes the hinges 56 to pivot from their full radial position to a position such that they are generally tangent to the periphery of cylinder 30 permitting secondary air to flow without impediment through secondary air passage 34. Rotation of shaft 86 in a counterclockwise direction will result in movement of the driving or actuator vanes 58 from passage full open to passage full closed position, depending upon the extent of such rotation. Thus, the burner supplies its own air for complete combustion when fired into chamber 14 regardless of whether the air pressure within the chamber 14 is positive or negative. The vanes vary the free area for the flow of secondary combustion air at the fire ring 22 such that the velocity of the air flow remains approximately the same between high fire and low fire even though the volume of air varies, due to the change in the effective area of the secondary flow path. Thus, the flame is stabilized and there is no tendency to lift or pull off the flame retention radial vane type diffuser at high fire, since the primary air remains under constant or set control because the control disc assembly 48 is preset and is not adjusted during normal burner operation.

Some control of combustion is achieved by modifying the configuration of fire ring 22 as illustrated more clearly in FIGS. 2 and 3. In FIG. 1, the ring is cylindrical in form, while in FIG. 2 the downstream end of ring 22 in this embodiment flares outwardly as at 96, while in the FIG. 3 embodiment, modified fire ring 22" terminates at its downstream end in a portion 98 which flares inwardly.

From the description of FIGS. 1 through 4, it is seen that the improved burner 10 of the present invention is so constructed that the air supply is controlled inside the blast tube 16 near the diffuser 50 so that maximum air pressure is delivered to the burner head to give pulsation free combustion under adverse conditions and regardless of pressure conditions within the combustion chamber 14. Further, primary air in passage 32 leading to the diffuser 50 is controlled separately from the secondary air within passage 34 which surrounds it and does not modulate with the secondary air. This gives a positive set of burning conditions to the diffuser 50 so that there is no tendency by modulation of the air stream for the fire to lift off the diffuser 50 or pulsate. The control for the primary air resides in the control disc assembly 48 which acts as an air supply damper for the diffuser 50, this being adjustably controlled by means (not shown) and which is adjusted only during initial assembly, resulting in modulation of the combustion air totally bymeans of vanes 58 and 60 or alternative forms of secondary modulation control within secondary air flow passage 34 during burner operation. In that regard, the vanes 58 and 60 are hinged at their radially inner sides and fall forward to provide a uniform air flow pattern and to permit the follower vanes 60 to move under the bias of the secondary air flow to follow the driving vanes 58.

Referring to FIG. 5, a second embodiment of the modulating arrangement for the secondary air flow is illustrated in conjunction with a burner generally similar to that of the embodiment of FIGS. 1 and 4, and like elements carry like numerical designations. In this respect, the burner 10 is comprised of a first cylinder 16 or blast tube which cooperates with a smaller diameter, second cylinder 30 to form a primary combustion air flow passage 32 within cylinder 30 and a secondary annular air flow passage 34 between cylinders 16 and 30. Fuel nozzle 36 again supplies fuel near the downstream end of cylinder 30 and in the vicinity of the diffuser 50, and within central opening 54 of that diffuser to effect mixture with the primary combustion air and ignition by creating a spark across electrodes 42 in similar fashion to the prior embodiment. A cylindrical fire ring 22 is employed at the downstream end of the cylinder or blast tube 16 in the same manner as the prior embodiment. What differs in this case, is the fact that the diffuser 50 is preferably provided with an integral diverging rim in the direction of air flow and away from diffuser vanes 52 and acting as a fixed radial projection within the secondary combustion air flow passage 34. The diffuser rim 110 does not extend completely across the annular secondary air flow path 34 but terminates with its peripheral edge 112 at some radial distance from the downstream end 114 of the outer cylinder 16 although coplanar therewith. Modulation of the secondary combustion air flow is achieved by the use of a third cylinder 116 which is mounted for axial shiftable movement with respect to cylinder 16 and in contact therewith. Axialshifting in the direction indicated by arrow 118 occurring -by extension of the control rods 120 coupled to the cylinder 16 at spaced circumferential positions by means of rivets 122. The downstream end or section 1240f the cylinder 116 is radially or obliquely inclined at an angle related to the angle or rim 110 and the length of the section 124 of cylinder 116 being such that when the cylinder 116 is in the axially retracted position, as shown in FIG. 5, in full lines, the secondary air flow passage 34 is completely closed off. However, axial shifting of the third cylinder 116 to the dotted line position in the direction of arrow 118 opens the secondary flow path passage permitting secondary air flow to mix with the primary air and fuel mixture, at the downstream end of the blast tube 16. Actuation of control rods 120 may be achieved in much the same manner as that of the embodiment of FIG. 1 and FIG. 4.

A third embodiment of the present invention is illustrated in FIGS. 6 and 7, again like elements being given like numerical designations. The blast tube or cylinder 16 concentrically surrounds the smaller diameter tube 30 and again forms secondary air flow path 34 between the cylinders while cylinder 30 defines the primary air flow path 32 leading to diffuser 50 which lies just downstream of nozzle 36 and electrodes 42. The fire ring 22 is fixed to the downstream end of the blast tube 16 in similar fashion to the prior embodiments.

The arrangement for modulating secondary air flow through secondary air flow passage 34 comprises in this case, a pair of angularly shiftable, annular plates identified generally at 120 and 122 respectively, the annular plate 120 constituting the downstream plate and being fixed to respective cylinders 16 and 30, at its inner and outer rims 124 and 126; the rims being fixed to the outer periphery of the cylinder 30 and the inner periphery of the cylinder 16 by conventional means such as welding, rivets, nuts and bolts or the like. The outer annular plate 120 is provided with a plurality of openings 126 of radially directed oriented openings 126 at circumferentially spaced positions, the openings preferably being formed by severing the annular plate 120 along three sides 128, 130 and 132 of the openings, and by bending the struck out segments 134 at right angles to the plane of the annular plate to define the opening 126. The struck out segments 134 form circumferentially spaced radial vanes which assist in controlling air flow into the area of the fire ring just downstream of the diffuser 50. The angularly shiftable upstream annular plate 122 which abuts the fixed plate 120 is L-shaped in cross section having an inner rim or flange 136 which has an internal diameter on the order of the outside diameter of cylinder 30 such that it acts as a bearing and may readily rotate about the circumference of cylinder 30 to a limited degree. The movable annular plate 122 is provided with openings, indicated at 138 in dotted line fashion, in FIG. 6, which are slightly narrower than the openings 126 formed within the fixed annular plate 120, but have the same radial length. Modulation of the secondary air flow through passage 34 is achieved by varying the effective cross sectional area of that flow path by angularly shifting the movable annular plate 122 relative to the fixed plate 120. This is achieved generally through the same mechanism illustrated in the embodiment of FIGS. 1 and 4, but in this case a single control rod 120 is operatively coupled to the annular plate 122 by means of a conventional bell crank mechanism 140 including a bell crank member 142 pivotably mounted to the cylinder 16 via mounting shaft 144 such that it rotates about the shaft axis and being connected to rod 120 by pivot connection 146 and being pivotably coupled to the annular plate 122 through pivot connection 148. Thus, shifting of the control rod in the direction of tis axis effects angular rotation to a limited degree of the upstream annular plate 122 relative to the downstream plate, closing off or opening the series of openings 126 carried by the fixed annular plate. This motion of course has no effect on the air flowing through the primary air flow passage 32 which exits from between the vanes 52 of the diffuser 50 and the central opening 54 of that diffuser.

The elements making up the improved burner and in particular the secondary control mechanism for the present invention are principally formed of metal as illustrated, it being necessary only that the elements be resistant to the high temperature experienced in the area of combustion.

From the above, it may be seen that the secondary air control arrangements for the liquid fuel burner of the present invention not only facilitates burner operation, but permits the burner to be manufactured in stock fashion for meeting a greater variety of heating needs, for instance, the improved burner may be employed in either fire box or scotch boilers with out alteration. The burner is fully modulating, under either high fire or low fire starts by simply varying the control covering the modulation of the secondary air flow passage. The blast tube length is not limited by the modulating arrangement. Pulsation free combustion occurs even under adverse conditions, since the secondary air supply is controlled inside the blast tube 16 near the diffuser 50 so that maximum air pressure is delivered to the burner head. Since the primary air to the diffuser is controlled separately from the secondary air which surrounds it and does not modulate with the secondary air, there is provided a positive set of burning conditions to the diffuser which do not change under modulation of the secondary air, and thus there is no tendency for the fire to lift off the diffuser or pulsate as a result of secondary air modulation.

What is claimed is:

1. In a modulating fuel burner for a furnace or the like, having a first cylinder defining a blast tube positioned within the furnace combustion chamber, and forming an axial combustion air flow path, a second cylinder of smaller diameter concentrically positioned within said first cylinder and dividing the air flow into an inner primary combustion air flow path within said second cylinder and an outer secondary combustion air flow path between said cylinders, a fuel nozzle axially positioned within the second cylinder for discharging fuel into the primary combustion air flow path at a flow rate dependent upon burner demand, and means for igniting the fuel air mixture downstream of said nozzle, the improvement comprising:

movable flow restriction means positioned between said first and second cylinders for varying the cross sectional area of the secondary combustion air flow path, and motor means operatively coupled to said flow restriction means for moving said flow restriction means to vary the secondary air flow only during burner operation in response to burner demand;

whereby, pulsation free combustion occurs under both high and low fire conditions and regardless of the pressure within the furnace combustion chamber,

and wherein said movable flow restriction means for varying the cross-sectional area of the secondary combustion air flow passage comprises; a plurality of circumferentially arranged hinged vanes within said secondary combustion air flow path and hinged to the periphery of one of the cylinders and means coupling said motor means to said vanes for varying the angular position of said vanes between a position in line with the direction of flow of the secondary air and a position at right angles thereto, said vanes being hinged to the outer periphery of said second cylinder, a ring member being fixed to the inner periphery of the first cylinder generally in line with the hinges for said vanes, and said hinges being of a radial length such that the radial outer ends of at least some of said vanes contacts said ring member when moved to a position generally at right angles to the direction of flow of said secondary air, to close off said secondary combustion air flow passage.

2. The fuel burner as claimed in claim 1, wherein said vanes comprise sets of alternating driving vanes and follower vanes with the edges of the follower vanes overlapping the edges of the driving vanes to the upstream side of the same, said means for varying the angular position of said vanes includes positive drive means operatively coupling said motor means to said driving set of vanes acting to pivot said driving vanes from a flow in line position to said position at right angles to the secondary air flow causing said follower vanes to move likewise to that position, in opposition to the secondary air flow, with said secondary air flow tending to return the vanes to an air flow in line position.

3. The fuel burner as claimed in claim 2, wherein said means for positively pivoting said driving vanes between said in line position and a position at right angles thereto comprises: an actuator shaft mounted at right angles, upstream of said second cylinder, and extending across said flow path, and wherein said motor means comprises a motor responsive to burner demand for rotating said actuator shaft, and a plurality of vane control rods are each pivotably coupled at one end to said actuator shaft and at the other end to a respective driving vane and movable in the direction of the longitudinal axis of the control rod in response to rotation of said actuator shaft by said motor to pivot said driving vanes between said positions.

4. The fuel burner as claimed in claim 1, wherein a fuel line is fluid connected to said fuel nozzle, said fuel line includes a flow control valve therein, and means operatively couple said motor means to said flow control valve such that operation of said motor means varies the flow rate of secondary air flow through said outer secondary combustion air flow path between the cylinders and varies the discharge of fuel from the fuel nozzle in response to burner demand during burner operation. 

1. In a modulating fuel burner for a furnace or the like, having a first cylinder defining a blast tube positioned within the furnace combustion chamber, and forming an axial combustion air flow path, a second cylinder of smaller diameter concentrically positioned within said first cylinder and dividing the air flow into an inner primary combustion air flow path within said second cylinder and an outer secondary combustion air flow path between said cylinders, a fuel nozzle axially positioned within the second cylinder for discharging fuel into the primary combustion air flow path at a flow rate dependent upon burner demand, and means for igniting the fuel air mixture downstream of said nozzle, the improvement comprising: movable flow restriction means positioned between said first and second cylinders for varying the cross sectional area of the secondary combustion air flow path, and motor means operatively coupled to said flow restriction means for moving said flow restriction means to vary the secondary air flow only during burner operation in response to burner demand; whereby, pulsAtion free combustion occurs under both high and low fire conditions and regardless of the pressure within the furnace combustion chamber, and wherein said movable flow restriction means for varying the cross-sectional area of the secondary combustion air flow passage comprises; a plurality of circumferentially arranged hinged vanes within said secondary combustion air flow path and hinged to the periphery of one of the cylinders and means coupling said motor means to said vanes for varying the angular position of said vanes between a position in line with the direction of flow of the secondary air and a position at right angles thereto, said vanes being hinged to the outer periphery of said second cylinder, a ring member being fixed to the inner periphery of the first cylinder generally in line with the hinges for said vanes, and said hinges being of a radial length such that the radial outer ends of at least some of said vanes contacts said ring member when moved to a position generally at right angles to the direction of flow of said secondary air, to close off said secondary combustion air flow passage.
 2. The fuel burner as claimed in claim 1, wherein said vanes comprise sets of alternating driving vanes and follower vanes with the edges of the follower vanes overlapping the edges of the driving vanes to the upstream side of the same, said means for varying the angular position of said vanes includes positive drive means operatively coupling said motor means to said driving set of vanes acting to pivot said driving vanes from a flow in line position to said position at right angles to the secondary air flow causing said follower vanes to move likewise to that position, in opposition to the secondary air flow, with said secondary air flow tending to return the vanes to an air flow in line position.
 3. The fuel burner as claimed in claim 2, wherein said means for positively pivoting said driving vanes between said in line position and a position at right angles thereto comprises: an actuator shaft mounted at right angles, upstream of said second cylinder, and extending across said flow path, and wherein said motor means comprises a motor responsive to burner demand for rotating said actuator shaft, and a plurality of vane control rods are each pivotably coupled at one end to said actuator shaft and at the other end to a respective driving vane and movable in the direction of the longitudinal axis of the control rod in response to rotation of said actuator shaft by said motor to pivot said driving vanes between said positions.
 4. The fuel burner as claimed in claim 1, wherein a fuel line is fluid connected to said fuel nozzle, said fuel line includes a flow control valve therein, and means operatively couple said motor means to said flow control valve such that operation of said motor means varies the flow rate of secondary air flow through said outer secondary combustion air flow path between the cylinders and varies the discharge of fuel from the fuel nozzle in response to burner demand during burner operation. 